Boriding is known to increase wear-resistance in metallic surfaces. Various methods of boronizing metallic surfaces are known. Such methods produce a boron layer on a metal surface. Typically, these methods utilize reactive boron species which diffuse into the metal surface. Such reactive boron species include gaseous diborane and boron trihalides, including BCl3 and BF3.
One method for boriding metallic surfaces is the “pack” method. In this methods, the boron source is in the form of a solid powder, paste, or in granules. The metal surface is packed with the solid boron source and then heated to release and transfer the boron species into the metal surface. This method has many disadvantages including the need for using a large excess of the boron source resulting in the disposal of excessive toxic waste.
Another method for boriding metallic surfaces utilizes a plasma charge to assist in the transfer of boron to the metal surface. Typically, plasma boronization methods utilize diborane, BCl3, or BF3 where the plasma charge is applied to the gaseous boron-containing reagent to release reactive boron species. See U.S. Pat. No. 6,306,225 and U.S. Pat. No. 6,783,794, for example. However, these methods utilize corrosive and highly toxic gases and are thus difficult to utilize on an industrial scale.
Plasma boriding processes have several advantages, including speed and localized heating of the substrate. This prevents the bulk metal in the borided piece from annealing, obviating additional heat treatments to restore the original microstructure and crystal structure. As a result, it is desirable to have plasma boriding processes that retain the advantages of plasma treatment while reducing the hazards and costs connected with noxious chemicals.